Modern sewer systems generally carry sewage in large diameter pipes, typically made from cement or similar material. These pipes generally are run underground. At the intersections of adjacent pipes, where it is desired to either change the course or path of the underground sewer pipe, change the slope or elevation of the sewer pipe, or merely provide access to the sewer pipe for future repair work, it is conventional to provide a manhole assembly.
A manhole assembly primarily includes a manhole base, an intermediate or riser section which is sometimes called a shaft pipe, and a top section which is frequently cone shaped and is normally designed to receive the manhole cover which is conventionally seen on city streets and sidewalks. The manhole assembly serves as the transition between adjacent sewer pipes. At least two sewer pipes fit within respective sidewall openings in the base section of the assembly, one functioning as an inlet pipe carrying sewage into the manhole assembly, and the other functioning as an outlet pipe carrying sewage from the manhole assembly. In some instances, there may be more than one inlet pipe.
Problems associated with manhole assemblies include concrete corrosion, leaking connections to inlet and outlet pipes, water leakage through the manhole cover into the manhole, and forming difficulties related to manhole base sections having channels. The present application includes a liner and lining system to overcome the corrosion problem, a pipe-to-manhole sealing assembly to prevent faulty pipe connections to the manhole, a water tight but gas pervious device just below the manhole cover to prevent excessive surface or storm water flow into the manhole, and two improved channel forming devices.
Most manhole structures are concrete, either cast in place or precast. However, concrete has been known to corrode in sewer systems primarily due to the acids contained in sewage and the gases dissipated therefrom. Concrete corrosion deteriorates manhole assemblies causing a need for expensive repairs and replacements. In addition to expensive repairs and replacement, corrosion of the concrete manhole structure causes street cave-ins which may result in traffic accidents and fatalities.
As discussed in the article "Corrosion Below: Sewer Structures" by Kenneth K. Kienow and Karl E. Kienow, corrosion in manhole interiors can be traced to two major causes. The first cause, referred to as "acid attack", is caused by low pH industrial waste discharged directly into the sewer system. Acid attack causes corrosion below the waterline of the sewage. The second cause referred to as "sulfide attack", causes corrosion above the waterline of sewage and occurs when sulfate in the sewage is converted to hydrogen sulfide gas, which is subsequently released into the air and deposited on the moist manhole wall, where bacterial action converts the gas to sulfuric acid which corrodes the manhole walls.
Corrosion of manholes is accelerated by sewage flow, and sewage and weather conditions. For example, a turbulent sewage flow can corrode a manhole significantly faster than a calm flow. Additionally, sewer stagnation can also increase manhole corrosion. The sewage pH strength and temperature, and the air current and humidity also affect corrosion of manhole walls. Corrosion can be reduced by minimizing turbulence. Turbulence is minimized by designing manholes where inlet and outlet pipes are at the same height and are not angularly disposed. However, minimizing turbulence does not fully and effectively prevent corrosion, and a better solution is needed.
Numerous attempts have been made to solve these corrosion problems. These attempts have included modifying the concrete mix, coatings that are sprayed, painted, or rolled onto the concrete surface, and liners that have integral locking projections cast into the concrete.
Attempts have been made to reduce the concrete corrosion by utilizing different compositions of the concrete, either by including additives or increasing the density. Different compositions have included the addition of one or more of the following, fly ash or pozzolanic materials, micro-silica, and high alumina cement. The effect on corrosion prevention by these compositions has been determined to be negligible at best, and detrimental at worst. Increasing the density of the concrete does help reduce corrosion. However, it is only a minor factor in corrosion prevention, and it may be costly.
Coatings which are sprayed, painted or rolled onto the manhole interior have not been successful either. These systems have been typically time consuming, costly and unreliable. Coatings depend on adhesion to stay in contact with the concrete. The unreliability of the coatings is due to the fact that their ability to adhere to the surface is very sensitive to improperly treated interior surfaces, surface moisture and weather conditions. Another disadvantage is that many coatings and adhesives are hazardous, requiring workers to be properly protected, the areas to be properly ventilated and the leftover materials used to be properly disposed of. Specific coatings which fall into this group include cementious, coal-tar epoxy, amine-epoxy, urethane, polymorphic-resin and polyester coatings. Specific problems associated with each are briefly described below.
Cementious coatings have sometimes failed because they contain cement paste which is chemically attacked by acid. Coal-tar epoxy coatings have had limited success, but environmental concerns have all but eliminated their use. Amine-epoxy coatings have failed because of chemical attack of the coating material itself and pinholes in the coatings. Urethane coatings have had many failures because they are very moisture sensitive. Polymorphic-resin coatings have been known to fail due to vapor pressure existing during the coating process. Lastly, polyester coatings have had problems because they do not tolerate dampness very well and they have also been known to have trouble handling the highly alkaline surface of good quality concrete.
Plastic liners having integral locking projections cast into the concrete have had success in reducing corrosion because the plastic protects the concrete from both acid and sulfide attack. The projections form a mechanical lock between the projections and the concrete.
FIG. 1 shows a prior art plastic liner 2 having integral T-shaped locking projections 4 as disclosed in U.S. Pat. No. 2,816,323 to Munger. Projections 4 are parallel to each other and each projection includes a leg segment 6 and a top segment 8. Liner 2 is extrusion molded in sheets and is taken to the manhole site subsequent to being formed. A sheet is rolled into a cylindrical shape corresponding to the pipe or cylindrical portion to be lined with its edges overlapping. The overlapping edges are then welded by the application of heat to the plastic. A mold for concrete is set-up and concrete is poured around liner 2 and inside the concrete mold. A mechanical lock is formed by the concrete settling between top segment 8 of projections 4 and the body of liner 2.
FIGS. 2-4 show alternatively shaped prior art parallel projections also disclosed in Munger, U.S. Pat. No. 2,816,323. FIG. 2 shows a projection 10 similar to the T-shaped projection 4, differing by having a bulbous shaped top segment 12 instead of a flat top segment 8, while FIG. 3 shows dovetail shaped projections 14 and FIG. 4 shows pairs of inwardly converging flanges 16, 18.
The prior art liners disclosed in FIGS. 1-4 have some disadvantages. Projections 4, 10, 14 or 16 only extend in one axial direction. This arrangement provides a mechanical lock along spaced parallel axes with the sections between projections 4, 10, 14, or 16 being susceptible to bulging, which increases the possibility that projections 4, 10, 14, or 16 may pull loose from the concrete. Additionally, in practice, these liners are highly plasticized which make them vulnerable to puncturing, cutting and tearing by sewer cleaning equipment.
Another prior art device is disclosed in U.S. Pat. No. 4,751,799 to Ditcher et al., in which a lining system utilizes curved liner sections. The sections are produced by vacuum thermo-forming plastic around a curved mold member, and each section composes a quadrant of a manhole area to be lined. The curved mold member includes a plurality of strips and holes. The holes permit the vacuum forming to occur while the strips act as part of the mold to form parallel T-shaped projections on the curved liner sections. Although the projections are described in the specification as being T-shaped, the projections actually resemble a vertical leg segment with a bulbous shaped top section. The sections are thereafter cooled and removed from the mold. The quadrants are then joined together and concrete is poured around the joined cylinder forming a mechanical lock with the projections.
The Ditcher liner system also has its disadvantages. First, the liner system is very labor intensive which increases the total manhole installation cost. Secondly, the projections are typically far apart which decreases the strength of the mechanical lock between the concrete and the liner. Additionally, the liner system is designed for manhole walls which makes it difficult to use such a system on a manhole floor.
Prefabricated plastic and fiberglass manholes have been introduced but they are not typically used in the industry because they are expensive, difficult to install, and in high ground-water areas have been known to float out of the hole without proper ballast or anchorage.
Thus a lined manhole assembly which has a strong mechanical lock and which effectively protects concrete manholes from corrosion is desired. Further, it is desirable that the liner not bulge after installation. It is also desirable to provide an inexpensive liner system which can cover essentially the entire manhole interior and which is not labor intensive.
In addition to manhole interior corrosion, fitting assemblies for attaching an inlet or outlet pipe to the manhole have also created problems. Differential settling of the manhole and the sewer pipe can break the pipe causing infiltration and exfiltration at the pipe to manhole connection. This problem occurs more frequently in areas where the soil conditions are unstable. A pipe joint located just outside the manhole tends to permit flexibility between the sewer pipe and the manhole and reduce the possibility of pipe breakage. Prior art devices, which include elastomeric gaskets and couplings, are intended to reduce these occurrences by providing a flexible, watertight connection between the manhole and the sewer pipe. Many prior art devices, however, have failed resulting in infiltration and exfiltration at the pipe-to-manhole connection. Further, many prior art devices use pipe clamps which are difficult to use and can cause failures.
Many existing pipe-to-manhole fitting assemblies do not permit high tolerances between the pipe and the hole in the manhole wall, subsequently causing difficult installations and cracked or broken pipes due to differential settlement between the pipe and the manhole. A pipe-to-manhole fitting assembly which permits high tolerances between the pipe and the hole in the manhole wall allows flexure of the pipe and differential settlement that otherwise would break the pipe. Thus, it is desirable to provide a pipe to manhole fitting assembly which is easy to install, eliminates leakage and provides high tolerances for pipe movement.
Another problem existing in sewer systems is the infiltration of rainwater into the sewer system through the manhole covers. During a rainy day, 3,000 to 12,000 gallons of rainwater can enter sewer treatment systems through the pickholes in an average sized manhole cover. This rainwater infiltration has been known to result in a flow increase of up to 40% of the volume handled by sanitary sewer systems overburdening treatment plants and creating contamination problems via overflows to waterways.
In many regions, the increased construction of streets, buildings and parking lots, has decreased available ground surface area which is necessary to absorb moisture from rain and snow. This has caused more drainage water to enter existing sanitary sewer systems, many of which are already operating at peak levels. By significantly preventing rainwater from entering into sanitary sewer systems through the manhole covers, many existing overburdened sanitary sewer facilities could operate at safe levels and sewer facilities already operating at safe levels can handle additional sewage capacity.
However, in preventing water leakage through the manhole cover, the manhole opening should not be totally sealed because when sewers are sealed gas-tight, the sulfide and corrosion problem is exacerbated. It is important that fresh air be drawn naturally into the sewer as sewage flow levels drop to minimize corrosion by reducing sulfide production and diluting sewer hydrogen sulfide gas concentrations. With no oxygen entering the manhole from the outside air, the sewer becomes septic and creates high levels of hydrogen sulfide gas. Further, it is important to reduce the buildup of pressure from the sewer gases inside the manhole because restricting the ability of hydrogen sulfide gas from exiting the manhole also increases corrosion.
U.S. Pat. No. 3,969,847 to Campagna et al., U.S. Pat. No. 4,650,365 to Runnels, and U.S. Pat. No. 4,919,564 to Neathery et at., disclose manhole inserts for installation directly underneath the manhole cover. These inserts prevent water inflow and permit internal sewer gas relief upon the internal pressure exceeding a fixed value. However, these inserts use valves or spring loaded members to permit the sewer gas relief, which increase the cost of the insert and have the ability to fail. More importantly, some of these devices are unable to permit fresh outside air from entering the manhole increasing sulfide production and sewer hydrogen sulfide gas concentrations. Therefore, an inexpensive and reliable valveless manhole insert preventing substantial water inflow and permitting internal sewer gas exit and outside air ingress is desired.
Another problem contemplated by this invention is the difficulty of forming manhole base sections with channels, which are required to accommodate one, two, three or more inlet pipes, and an outlet pipe, at numerous different angular configurations.
Inlet and outlet sewer pipes do not abut within the riser. Instead the floor of the manhole base section includes a channel or channels which carry the flow from the inlet or inlets to the outlet. The channels are merely U-shaped troughs which connect the inlet pipe or pipes to the outlet pipe.
It is important for the channels to be properly formed in the manhole assembly to assure smooth flow and maximize the flow rate and minimize turbulence through the manhole assembly. Most manhole base sections provide a channel which extends along a diameter of the manhole floor. The inlet sewer pipe merely rests in the manhole base section at one end of the channel and the outlet sewer pipe rests at the diametrically opposite end, with the channel serving to connect the two aligned pipes. The inlet and outlet pipes thus form a straight line, or in other words, the outlet pipe is disposed at 180.degree. with respect to the inlet pipe. These base section floors with "straight line" channels can be readily made at a factory and shipped to a job site for easy use. These assemblies typically are formed using a so-called one-pour or single pour technique in which mold members form and define the base section having a wall and a floor with a channel. However, in a significant number of situations, it is required that the inlet sewer pipe be angularly disposed with respect to the outlet sewer pipe. Thus, the channel would not extend straight across the manhole along the diameter, but instead, must be curved or angled to connect, for example, an inlet sewer pipe which enters a manhole at the twelve o'clock position and an outlet sewer pipe which exits the manhole at the three o'clock position or disposed at 90.degree. with respect to the inlet.
It is not practical to provide a channel forming apparatus for each angular relationship which may be required at different sites along the sewer line. Typically, angled channels in manhole base section floors have been formed in two stages, referred to as a two-pour technique. Part of the manhole base section floor without the channel is formed at the factory and delivered to the job site. The base section floor without the channel is positioned in the ground at the site of the manhole assembly and a forming apparatus for forming a channel is lowered into the hole along with a workman who manually forms the channel and completes the floor. This process is time-consuming, subject to difficult quality control problems because of the differing nature of each channel formed in each manhole base section floor, and is quite costly.
Various devices for forming channels in base section floors at various angular positions in a manhole assembly have been proposed. U.S. Pat. 4,484,724 to Srackangast discloses an apparatus for forming curved channels which uses a plurality of complementary wedge-shaped forming elements. The correct number and size of elements must be selected and manually fixed together. This is a time-consuming and laborious process and subject to a substantial amount of trial and error in selecting the correct elements. Similarly, U.S. Pat. Nos. 4,103,862 to Moore; and 4,422,994 and 4,685,650 to Ditcher disclose channel forming devices which are difficult and time-consuming to use. Accordingly, there is a need for a channel forming assembly for forming a channel or channels in a manhole base section floor accommodating one, two or three incoming sewer pipes having any desired angle between each incoming sewer pipe and the outgoing sewer pipe over a range of approximately 90.degree. to 270.degree. which is simple and economical to use.